By transmitting laser light into the atmosphere, a prior art laser radar apparatus receives laser light which has a Doppler frequency shift due to the drift speed of scatterers in the atmosphere, and carries out heterodyne detection of the laser light and local light so as to detect a Doppler signal.
The prior art laser radar apparatus then acquires the drift speed of the scatterers from the frequency of the Doppler signal.
As mentioned above, although the prior art laser radar apparatus employs a method of detecting the Doppler shift frequency for the carrier frequency of the laser light, it is known that this method exhibits weak coherency of the Doppler signal. In other words, it is known that the coherent time of the Doppler signal is short. For example, when the received light is light scattered from aerosols in the atmosphere, it is known that the coherent time of the Doppler signal is of the order of microseconds (μs).
Thus, when the coherent time of the Doppler signal is short, an incoherent integral of the Doppler signal is carried out so as to obtain an improvement in the S/N ratio of the received light, but it is known that it is difficult to effectively improve the S/N ratio of the received light even if an incoherent integral of the Doppler signal is carried out.
In order to acquire a Doppler signal having a long coherent time, it is known that an intensity modulation of the light signal is carried out by using a frequency (for example, a modulation frequency which falls within a microwave frequency band) lower than that of the light signal, and what is necessary is just to detect the Doppler frequency for this modulation frequency.
Following patent references 1 to 3 disclose laser radar apparatus for carrying out an intensity modulation of a light signal by using a modulation frequency which falls within a microwave frequency band.                Patent reference 1 Japanese patent application publication No. 59-150299        Patent reference 2 Japanese patent publication No. 51-29032        Patent reference 3 Japanese patent application publication No. 2-25786        
A problem with the prior art laser radar apparatus constructed as mentioned above is that while it can acquire a Doppler signal having a long coherent time, when the S/N ratio of received light is low, there is a possibility that the prior art laser radar apparatus is not able to detect the quality of scatterers in the atmosphere since it has no means for improving the S/N ratio of received light.
For example, when the scatterers are aerosols in the atmosphere and the prior art laser radar apparatus receives a light signal scattered from aerosols and detects the drift speed (i.e., the air velocity) of the aerosols, the S/N ratio of the received light signal deteriorates greatly as compared with the case where the scatterers are hard targets, such as automobiles.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a laser radar apparatus that can detect the quality of scatterers in the atmosphere even when the S/N ratio of received light is low.